Lettuce is currently packaged in a non-stretchable heat resistant film. The film structure includes a layer of polyester bonded to a layer of polyethylene. This film offers low moisture permeability which is desirable in packaging lettuce and similar produce. However, it is also low in gas permeability, which reduces the shelf life of chopped lettuce when oriented heat set polyester is used.
Other heat set films typically used in this application have oxygen transmission rates of less than 5000 cc/meter.sup.2 at 1 atmosphere 24 hours at 73.degree. F.
With the packaging of lettuce and similar produce, it is desirable to provide a packaging material with a high gas permeability, preferably greater than 5000 cc per square meter, yet with low moisture transmission rates in order to resist loss of moisture from the food product.
A packaging material with these properties is disclosed in U.S. Ser. No. 240,994 now U.S. Pat. No. 4,935,271, assigned to W. R. Grace & Co.-Conn. The laminate comprises a first perforated film having propylene homopolymer or copolymer, and a second film bonded to the perforated film including a bonding layer of ethylene vinyl acetate copolymer and an outer heat sealable layer.
While this film provides the high gas permeability and low moisture permeability, as well as stiffness and toughness required in the laminate for packaging produce, it is desirable to provide a film useful for packaging produce as well as other materials and having good heat resistance while maintaining the breathability of the film.